The present invention relates to a protein which is induced in the course of the soaking of mature seeds of plants belonging to species of the families Chenopodiaceae, Compositae, Labiatae and Cucurbitaceae, and to its use as a molecular marker of the soaking phase involved in the germination of these seeds and in the osmoconditioning and hydroconditioning processes of these seeds, called pre-germination.
Germination is a complex developmental process, for which only few exact molecular data are currently available. This developmental programme, in the course of which the cells of the embryo change from a state of dormancy to a state of intense metabolic activity, is set up essentially during the soaking phase. In the physiological sense, it stops when the plantlet which bursts forth ruptures the seed coats (Bewley & Black, 1983).
The main techniques used for defining the germination ability of seeds make use of traditional germination tests, that is to say the germination percentage is measured for a given batch of seeds under defined conditions at different points in time after sowing (temperature, humidity, light, substrate). The criterion which is generally used for quantitatively determining germination is the rupturing of the seed coat by an organ of the plant which bursts forth. Most of the biochemical markers described to date are correlated with this phase. Thus, they are, strictly speaking, not markers of germination, but rather of the initial growth phases (Fincher, 1989). It must be noted that the initial soaking phase (that is to say, the uptake of water) is reversible up to a certain point. After a controlled hydration of the seeds, they may be dried while preserving their biological integrity and their germination capacity. From the point at which the plantlet appears, commitment to growth becomes irreversible. In fact, dehydration at this stage irreparably leads to the death of the plantlets (Bewley & Black, 1983).
The pre-germination methods which have been developed by seed companies are based on the reversibility of the initial soaking phase. In general, the seeds are hydrated in a controlled manner and then dried (Coolbear et al., 1987; Karsen et al., 1989; Tarquis & Bradford, 1992). These methods add real extra value to the seeds because they allow
1) batches of seeds to be homogenized in relation to germination, PA1 2) an appreciable saving of time to be achieved for emergence after sowing, since a certain number of biochemical processes, which are required for accomplishing germination, will already have been taking place during the treatment, PA1 3) an improvement of the germination quality of old batches of seeds, probably on account of the fact that the repair mechanisms of the biological structures which have been damaged during the final maturation of the seeds, namely during storage, are set up during the treatment.
The optimization of such processes currently rests solely on carrying out germination tests, which require several days of experimenting. Moreover, if the treatment fails (batches of seeds are by nature heterogeneous, it is therefore necessary to optimize the treatment for each batch), the batch is lost. The possibility of continuously monitoring the soaking phase by means of an easily detectable molecular marker would therefore represent considerable progress which would allow the pre-germination treatment to be adapted to each batch of seeds.